While sound waves are conducted from one medium to another medium, the sound waves will be reflected, refracted or scattered in the interface between the two mediums, which depends on the difference of the acoustic impedances of the two mediums. Normally, the greater the difference of the acoustic impedances, the more intense the reflection. Some of the sound waves will be refracted in the medium behind the interface. If the dimension of the interface is smaller than the wavelength, scatter will occur. Therefore, the measurement of acoustic impedances is indispensable in designing and fabricating the products involving conduction or reception of sounds, such as earphones and artificial ears.
B. H. Song and J. S. Bolton proposed a paper “A transfer-matrix approach for estimating the characteristic impedance and wave numbers of limp and rigid porous materials” in J. Acoust. Soc. Am. 107, 1131-1152 (2000). The paper disclosed a measurement method of a dual-port measurement system. The dual-port measurement system comprises two impedance tubes, i.e. a first impedance tube and a second impedance tube. The terminal of the first impedance tube is the input end of the whole dual-port system; the start end of the second impedance tube is the output end of the whole dual-port system. The tested object is arranged between the input end and the output end. Each impedance tube has two microphones. The relationship between the sound pressure vectors measured by the microphone array and the transfer matrix are used to work out the incident waves and reflected waves of the first impedance tube and the second impedance tube. The incident waves and reflected waves are used to obtain the sound pressures and the volume velocities at the input end and the output end. Then, suppose the tested object satisfies symmetry and reciprocity, and use the relational equation of the output end and the input end to work out the transfer matrix and obtain the acoustic impedance of the tested object.
However, the abovementioned measurement method only applies to the test objects simultaneously satisfying symmetry and reciprocity and only adapts to a single algorithm. Thus, the conventional technology is only suitable to a single type of tested objects. Therefore, the application thereof is limited and inconvenient.